1. Field of the Invention
The invention is related to the field of magnetic disk drives, and in particular, to compensating for the flying height of sliders in magnetic disk drives, such as due to changes in environmental conditions.
2. Statement of the Problem
Many computer systems use magnetic disk drives for mass storage of information. Magnetic disk drives typically include one or more sliders having a read head and a write head. An actuator/suspension arm holds the slider above the surface of the magnetic disk. When the magnetic disk rotates, an air flow generated by the rotation of the magnetic disk causes an air bearing surface (ABS) side of the slider to fly a particular height above the magnetic disk. As the slider flies on the air bearing, a voice coil motor (VCM) moves the actuator/suspension arm to position the read/write head over selected tracks of the magnetic disk. The read/write head may then read data from or write data to the tracks of the magnetic disk.
As the density of magnetic disks increases, it has become more important to precisely control the clearance or spacing between the read/write head and the surface of the magnetic disk. The clearance between the read/write head and the surface of the magnetic disk depends on the flying height of the slider. The flying height of the slider is controlled by the shape of the ABS and the rotational speed of the magnetic disk. As the magnetic disk rotates, the slider rides on an air flow at a flying height determined by the shape of the ABS of the slider. Disk drive manufacturers have developed desired ABS shapes that allow for desired flying heights, such as in the range of 8-12 nanometers.
Disk drive manufacturers may want to decrease the clearance between the read/write head and the surface of the magnetic disk without having to further decrease the flying height of the slider and risk having the slider contact the magnetic disk (referred to as head-to-disk contact). One way to decrease the head-to-disk clearance is to cause a protrusion of the read/write head toward the surface of the magnetic disk. The read/write head is fabricated from materials that are different than the rest of the slider body. These materials expand and contract due to temperature at different rates than the slider body. Disk drive manufacturers take advantage of these material properties by embedding one or more heating elements in the read/write head or proximate to the read/write head. A heating power may then be applied to the heating element(s) to generate a protrusion of the read/write head from the ABS of the slider. Controlling the protrusion of a read/write head through the application of a certain heating power to the heating element is referred to herein as Thermal Flying-height Control (TFC).
One problem with present disk drives is that the flying height of a slider may decrease when the disk drive is put into operation under certain conditions. For example, the flying height of a slider is sensitive to changes in environmental conditions, such as altitude changes, temperature changes, etc. When the disk drive is at the manufacturer, the flying height of the slider is determined at sea level using algorithms. The electrical channel function of the disk drive provides flying height measurement capabilities either directly or indirectly by evaluating the Wallace Spacing Loss algorithm on a readback signal to determine if the flying height of the slider is acceptable. The initial flying height that is measured by the manufacturer during calibration is referred to herein as an expected or desired flying height. When the disk drive is subsequently put into operation, different environmental conditionals may cause a reduction in the expected flying height of the slider. For example, at higher elevations, the flying height of a slider may lose 2-3 nanometers (using a typical value of 10 nanometers obtained at sea level), which may represent a loss of approximately 20-30% of the expected or desired flying height. At higher temperatures, a similar change in flying height may be observed. If the flying height of a slider is allowed to change by 20-30% due to changes in environmental conditions, then there is an increased risk of head-to-disk contact and a loss of data. If altitude and temperature effects were to be combined, then the flying height loss may be closer to 40-50% (loss of 4-5 nanometers from the typical 10 nanometers obtained at sea level). This unfortunately results in disk drives having lower reliability.